Gas discharge lamps for use in video projection applications are usually so-called AC-operated short-arc lamps, which, to achieve a high optical imaging quality, provide an arc length of about 1 millimeter, as a result of which there are special requirements on the stability of the electrode geometry. Lamps of this sort are identified, depending on the manufacturer, as P-VIP lamps or UHP lamps, for example. A typical operating method of a lamp of this sort consists in the injection of a current, with an amplitude that remains constant or whose level is modulated, into the lamp, while the current direction is reversed at particular times by a commutation device. Depending on the projection technology, it is necessary, in addition to these requirements for electrode stabilization, to match the mode of the lamp's operation closely to the customer's specified application. In DLP (Digital Light Processing) projectors in particular, precise synchronization with the color wheel that is usually used there must take place. This color wheel rotates in the beam path of the projector, and includes a plurality of color segments to which different colors are assigned, and each of which can have a different length. Different colors of a video image are in this way projected in sequence. Modern DLP projectors furthermore have an adjusting facility which permits an individual configuration of the lamp current levels in the individual color segments (Unishapem). Through this, for example, a higher brightness can result from an increase in the white segment, or a better color reproduction through appropriate adjustment of the individual colors to one another. Typically, a customer can define a plurality, for example three to seven, current curves for different sets of brightnesses in the individual segments, and can store these in a so-called EEPROM memory.
Expediently, a commutation is made at the transitions between the individual color segments of the color wheel, at the so-called spokes. The drop in brightness that is entailed by the commutation of the current is not then perceptible to the user, since the mixed colors arising at the spokes are in any case masked by the projector or, for example, are used in order to increase the proportion of white light (so-called spoke light recapture). In addition, so-called maintenance pulses, which represent increases in current arranged immediately before a commutation, advantageously affect the development or reshaping of an electrode tip. The curve of the amplitude of the lamp current and the pattern of the commutations thus have a fundamental effect on the behavior of the electrodes of the discharge lamp.
The electrode behavior for all lamps of a particular lamp type is not identical here, and in addition the electrode geometry also changes throughout the service life. Variations of the parameters of a lamp type, for example in the context of manufacturing tolerances, are not taken into account by the prior art; a current waveform that optimally operates a first lamp of a particular lamp type fresh from the factory can be less well-suited to another lamp of the same type. In this context, optimally operating means achieving a maximum useful life of the lamp.
Typically, a discharge lamp of this sort is operated in a power-regulated mode, meaning that a constant mean power is fed to the lamp. Due to the unavoidable electrode burn-back over the service life, the effective distance between the two electrodes rises, as does therefore also the burn voltage, as a result of which the current through the discharge lamp correspondingly falls while maintaining an average lamp power. The changed current itself again has an effect on the electrode geometry and the electrode reshaping during the service life.
In terms of a stable lamp operation, a local peak is formed at the starting point of the arc on each of the two electrodes, whereby jumping of the arc on the electrode surface is prevented. The shaping of the peak here is to be kept in a suitable surface area, within which the electrode tip is just molten at the starting point of the arc. Since the electrode is subject to greater thermal stress when acting as an anode than when acting as a cathode, it is necessary to ensure an equal thermal stress on the two tips through an appropriate temporal sequence of the commutations. The formation of the electrode tips can be positively affected by a suitable shaping of the current waveform, in particular if the temporal behavior of the current waveform, and the cyclic melting and re-cooling of the tip material associated with it, leads to a transport of material from regions behind the tip forwards into the tip.
In this connection, EP 2 168 408 B1 discloses a method for driving a gas discharge lamp, wherein an operating frequency of the lamp is switched from a first frequency value to a second frequency value, whereby in a first operating mode a growing tip results on an electrode of the lamp, and in a second operating mode the tip on the electrode is at least partially re-melted.
A method for operating a gas discharge lamp is known from DE 10 2009 006 338 A1, in which a repeated application of a DC phase takes place with a predetermined temporal spacing.
According to the known prior art, however, it is assumed that all the lamps change in the same, predictable manner as their service life continues. Experience shows, however, that this is not the case.
It is therefore desirable to achieve an improved lifetime performance.